Nodulation and Genomic Capacity of a Novel High-Latitude Bradyrhizobium japonicum HLNEAU001

Abstract

Rhizobium species are essential symbionts of legumes, participating in nitrogen fixation and having an important impact on the protein content of soybeans. In this study, we isolated Rhizobia from the soybean cultivar Suinong14 growing in a soybean field in Harbin (45.75° N, 126.53° E), which is situated within one of the three largest black soil belts in the world. One Bradyrhizobium japonicum strain (High-Latitude Northeast Agricultural University 001, HLNEAU001) was identified. Two hundred fifteen core soybean germplasms from Northeast China were screened for nodule traits, with the nodule number ranging from 0 to 95. Phylogenetic analysis showed that B. japonicum HLNEAU001 is closely related to B. japonicum USDA6 and B. diazoefficiens USDA110. A nodulation capacity analysis showed that B. japonicum HLNEAU001 formed more nodules than B. japonicum USDA6 or B. diazoefficiens USDA110, with 30 soybean germplasms being assayed for each strain. Using a draft genome sequence of B. japonicum HLNEAU001 to compare the genomic differences between B. japonicum HLNEAU001, B. japonicum USDA6 and B. diazoefficiens USDA110 showed that the three strains contain 5790 core genes. Because B. japonicum HLNEAU001 and B. japonicum USDA6 exhibited collinearity, the genomic differences between these two strains were further analysed. In addition to type IV and type VI secreted proteins, we hypothesize that type III effectors are the key factors underlying the nodulation capacity differences between B. japonicum HLNEAU001 and B. japonicum USDA6. The results of this study indicate that B. japonicum HLNEAU001 is a distinctive cold-region, slow-growing Rhizobium strain that is capable of effective symbiotic nitrogen fixation in cold regions and black soil and may play a pivotal role in sustainable agricultural production.